Set m_inheritBgCol flag in wxPanel with a background bitmap.

Set m_inheritBgCol flag in wxPanel with a background bitmap.

This ensures that the panel background is used by its transparent children
under wxMSW even when the panel itself is inside a wxNotebook with themed
background: making InheritsBackgroundColour() return true prevents the
notebook from overriding our background.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67304 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775